There is given an ultrasound diagnostic apparatus configured to utilize photoacoustic waves. Photoacoustic waves are ultrasound waves that are generated when, for example, a pulse laser beam (electromagnetic waves) irradiating a body from the outside is absorbed by tissue inside the body. Photoacoustic waves are generated in a particular type of tissue in a body, and information of photoacoustic waves can therefore be utilized in the imaging of body tissue. Sound waves, ultrasound waves, photoacoustic waves, and the like that are called acoustic waves are herein represented by ultrasound waves in some places.
An ultrasound probe can be used to detect ultrasound waves generated inside a body. An ultrasound transducer is disposed inside an ultrasound probe to convert ultrasound waves into electric signals. While ultrasound transducers that utilize the piezoelectric effect have been hitherto in use, capacitive transducers are being actively researched and developed in recent years. In an example of capacitive transducers, two electrodes provided so as to sandwich a space that is called a cavity and that is kept in a substantially vacuum state form a cell, at least one cell forms an element, and at least one element is included in the capacitive transducer. One of the two electrodes is fixed to a membrane, and the thus built vibration membrane is held in a manner that allows the membrane to vibrate. Vibrations of the vibration membrane from ultrasound waves change the distance between the two electrodes, and the change in distance changes the capacitance. When a voltage is applied between the two electrodes in advance, the change in capacitance can be taken out as a current signal. This is an example of the principle of ultrasound reception operation. Capacitive transducers that are manufactured by applying a semiconductor micromachining technology are called capacitive micromachined ultrasonic transducers (CMUTs). An ultrasound probe having a plurality of densely packed elements is suitable for image quality enhancement. Such ultrasound probe is demanded of ultrasound diagnostic apparatus.
In the technical field described above, an acoustic impedance matching layer, which matches acoustic impedances (makes the acoustic impedance values close to one another) of all or some substances between the subject and the element, is provided above the element in some cases. A light reflecting layer is sometimes provided above the element to reduce noise that originates from light.